Field of the Disclosure
The present disclosure generally relates to a bulge-resistant coke drum.
Description of the Related Art
Delayed petroleum coking is a process in which a petroleum fraction is heated to a temperature at which it thermally decomposes to provide a solid coke product and a hydrocarbon distillate product. In general, a liquid petroleum feed stock is first distilled until the lighter ends have been recovered and a heavy residuum remains. This heavy residuum of heated pitch and cat-cracked heavy or cycle oil is charged to the bottom of a pressure vessel called a coke drum.
In the coke drum, the heavy residuum is further heated to one thousand degrees Fahrenheit or more and undergoes extensive and controlled cracking and coking under high-pressure conditions. A cracked lighter product rises to the top of the coke drum in a process called steam stripping and is drawn off. A heavier product remains and cracks to coke, a solid, coal-like substance. The coke is usually purged with steam to remove any remaining volatile components. After the cracking and coking process is complete, quench water is introduced and high-pressure water jets are used to cut away and remove the coke. The water reduces the temperature in the drum to around two hundred degrees Fahrenheit or less before a new cycle begins.
In order to increase production speed, the quenching operation is often done as quickly as possible. Unfortunately, the faster the coke is quenched, the greater the wear and tear on the drum. One of the primary causes of this wear and tear is that the steel plate and the weld material joining the rings of plate that form the drum have different yield and creep strengths. In the vicinity of the circumferential weld sites, thermal cycling causes progressively increasing permanent strains, eventually leading to bulging and cracking and often bringing to an end the useful life of the drum.
U.S. Pat. No. 6,193,848 (FNI: Shockley, A′ee: CB&I) discloses a coke drum made by a plurality of vertical plates bent such that widths thereof are curved and adjoined by vertical welds. Unfortunately, due to manufacturing limitations, the length of the plates is limited to forty feet which is less than the typical length of modern coke drums such that the drum is left with at least one circumferential weld. Further, the coke drum requires extensive modification to the manufacturing process for conventional coke drums leading to a significant increase in cost and effort.
There remains a need for a coke drum that can better withstand extreme thermal cycling.